This is the law in California
Cal. Penal Code § 836 (D)
(d) Notwithstanding paragraph (1) of subdivision (a), if a suspect
commits an assault or battery upon a current or former spouse,
fiance, fiancee, a current or former cohabitant as defined in Section
6209 of the Family Code, a person with whom the suspect currently is
having or has previously had an engagement or dating relationship,
as defined in paragraph (10) of subdivision (f) of Section 243, a
person with whom the suspect has parented a child, or is presumed to
have parented a child pursuant to the Uniform Parentage Act (Part 3
(commencing with Section 7600) of Division 12 of the Family Code), a
child of the suspect, a child whose parentage by the suspect is the
subject of an action under the Uniform Parentage Act, a child of a
person in one of the above categories, any other person related to
the suspect by consanguinity or affinity within the second degree, or
any person who is 65 years of age or older and who is related to the
suspect by blood or legal guardianship,
a peace officer may arrest
the suspect without a warrant where both of the following
circumstances apply:
(1) The peace officer has probable cause to believe that the
person to be arrested has committed the assault or battery, whether
or not it has in fact been committed.
(2) The peace officer makes the arrest as soon as probable cause
arises to believe that the person to be arrested has committed the
assault or battery, whether or not it has in fact been committed.
(e) In addition to the authority to make an arrest without a
warrant pursuant to paragraphs (1) and (3) of subdivision (a), a
peace officer may, without a warrant, arrest a person for a violation
of Section 25400 when all of the following apply:
(1) The officer has reasonable cause to believe that the person to
be arrested has committed the violation of Section 25400.
(2) The violation of Section 25400 occurred within an airport, as
defined in Section 21013 of the Public Utilities Code, in an area to
which access is controlled by the inspection of persons and property.
(3) The peace officer makes the arrest as soon as reasonable cause
arises to believe that the person to be arrested has committed the
violation of Section 25400.
Here is a link for other states
http://www.americanbar.org/content/...rest_Policies_by_State_11_07.authcheckdam.pdf
Notice some say that there is a "mandatory arrest" policy and some say up to the discrestion of the officer.
So whether it is absolutely mandatory may be up to debate but several states say it is ok to make the arrest without the warrant.